1. Field of Invention
The present invention relates to a method for making an anti-reflection film of a solar cell and, more particularly, to a method for using SiCNO:Ar plasma to passivate a poly-silicon laminate to fill dangling bonds of silicon on the surface and at internal grain boundaries, thus providing an anti-reflection film of SiCN/SiO2 for a solar cell.
2. Related Prior Art
Most silicon-based solar cells are made in low-temperature processes based on plasma-enhanced chemical vapor deposition (“PECVD”). An amorphous-silicon or microcrystalline-silicon film is coated on a glass, aluminum, silicon, stainless steel or plastic substrate. A back contact is made of aluminum, gold, silver or transparent conductive oxide such as indium-tin oxide (“ITO”) and zinc oxide.
The primary advantage of the low-temperature processes is the wide variety of materials that can be used to make the substrates. However, they suffer drawbacks such as defective silicon films, low photoelectrical conversion efficiencies and low light-soaking stability.
In the PECVD, while coating the microcrystalline silicon film, a silicon material is highly diluted in hydrogen. For example, [H2]/[SiH4]>15. That is, the concentration or flow rate of H2 is more than 15 times as high as that of SiH4. The problems with the PECVD include a low growth rate of the film, a long process and a high cost.
As discussed above, there are problems with the PECVD-based low-temperature processes for making poly-silicon thin-film solar cells. Firstly, there are many defects in the silicon films. Secondly, the photoelectrical conversion efficiencies are low. Thirdly, the light soaking stabilities are low. Fourthly, the growth rates of the films are low. Fifthly, the processes are long. Sixthly, the costs are high.
To make the poly-silicon solar cells, there are other techniques such as solid phase crystallization (“SPC”) and aluminum-induced crystallization (“AIC”). The SPC is based on the PECVD. An amorphous silicon film is deposited, intensively heated and annealed at a high temperature. Thus, a poly-silicon film with a grain size of 1 to 2 micrometers is made.
In the AIC as shown in FIGS. 5 to 9, an aluminum film 42 is coated on a substrate 41. An amorphous silicon film 43 is coated on the aluminum film 42 based on the PECVD and annealed at a temperature of about 575 degrees Celsius for a long time to form a seed layer 44. Then, it is subjected to an epitaxial process such as the PECVD or an electron cyclotron resonance chemical deposition (“ECR-CVD”) to make a poly-silicon film 45. The resultant grain size is about 0.1 to 10 micrometers. The AIC however involves many steps and takes a long time. The cost is therefore high.
The present invention is therefore intended to obviate or at least alleviate the problems encountered in prior art.